White film for surface light source reflecting members

ABSTRACT

A white film for a reflecting structure for surface light sources, which contains voids inside it and has a light stabilizer-containing coating film formed on at least one surface of it, is aged little with time and its brightness lowers little even when used for a long time. The white film ensures good image quality of display screens for a long time, and it is favorable to reflecting sheets and reflectors in edge light-type and direct back light-type surface light sources for liquid crystal display screens.

TECHNICAL FIELD

This disclosure relates to improvements on white film for a reflectingstructure for surface light source. More precisely, the disclosurerelates to a structure of white film which is used in reflecting sheetsand reflectors for edge light-type and direct back light-type, surfacelight sources for liquid crystal display screens, and of which thebrightness decreases little when used for a long time.

BACKGROUND

For lighting units for liquid crystal display screens, widely used is anedge light system in which a cold cathode-ray tube serving as the sourceof light is disposed at the edge of a light guide plate (JP-A 62104/1988). In the lighting system of the type, a reflector is disposedaround the cold cathode-ray tube for increasing the lighting efficiencytherein, and a reflecting sheet is disposed below the light guide platefor efficiently reflecting the light having been diffused through thelight guide plate toward a liquid crystal display screen. In the system,these have the function of reducing the loss of light from the coldcathode-ray tube and increasing the brightness of the liquid crystaldisplay screen. However, the edge light system could not increase thebrightness of recent wide screens of liquid crystal TVs, for which,therefore, a direct back light system is being employed. In the directback light system, cold cathode-ray tubes are aligned in parallel belowa liquid crystal display screen, and they are above a reflecting sheetin parallel with each other. The reflecting sheet may be flat or may besemi-circularly shaped to partly cover each cold cathode-ray tube.

For use in the surface light source for such liquid crystal displayscreens, the reflector and the reflecting sheet (these are genericallyreferred to as a reflecting structure for surface light source) arerequired to have high reflectivity, for which generally used are filmswith white dye or pigment added thereto or films with fine voids thereineither alone or as laminated with any of metal or plastic plates. Thefilms with fine voids therein are widely used as their ability toimprove screen brightness and uniformity is good. They are disclosed in,for example, JP-A 322153/1994 and 118433/1995.

The recent expansion of the use of liquid crystal display screens isremarkable. For example, they are being widely used not only inconventional notebook-size personal computers but also in desk-toppersonal computers, liquid-crystal TVs, mobile telephone displays,various game computers, etc. With the expansion of their use, thescreens are desired to have increased brightness and increased fineness,for which the power of the lighting source is increased and the numberof the light source lamps is increased. In addition, for satisfying therequirement of long-term lighting of wide screens of liquid crystal TVs,they are required to have more increased brightness and durability. Inparticular, when the screens are lighted by a direct back light system,they shall directly receive the light from the light source. For these,the durability of the reflecting sheet is required to be higher.However, reflectors and reflecting sheets of conventional film areproblematic in that the film is aged and yellowed when used for a longtime and therefore its reflectivity is lowered with time and the screenbrightness is after all lowered.

It could therefore be advantageous to provide a reflecting structure forsurface light source of which the brightness decreases little with timeeven when used for a long time and which ensures high-quality images fora long times.

SUMMARY

We provide a white film for surface light source reflecting structures,which contains voids inside it and has a light stabilizer-containingcoating film formed on at least one surface of it.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a vertical cross-sectional view showing the outline structureof a device for measuring the brightness of the surface light source inthe invention.

DETAILED DESCRIPTION

The white film is not specifically defined so far as it is apparentlywhite, including, for example, thermoplastic films with any of organicor inorganic dye or fine particles added thereto; films formed by mixinga film-forming resin component and a resin not miscible with it, and/ororganic or inorganic particles, melt-kneading the resulting mixture, andstretching it at least in one direction to thereby make the film havefine voids therein; foam films formed through melt extrusion withfoaming particles added thereto; and foam films formed through foamingextrusion with a vapor such as carbon dioxide introduced thereinto.Preferred are films formed by mixing a film-forming resin component anda resin not miscible with it, and/or organic or inorganic particles,melt-kneading the resulting mixture, and stretching it at least in onedirection to thereby make the film have fine voids therein, as theirreflectivity is higher and their brightness is higher. More preferredare composite films formed through coextrusion of laminating an organicor inorganic fine particles-containing thermoplastic resin film on atleast one surface of a film that contains fine voids therein, followedby stretching the laminate film to thereby make it have finer voids inthe surface layer than in the inside thereof.

The thermoplastic resin to form the film is not specifically defined sofar as it forms films through melt extrusion. Preferred examples arepolyesters, polyolefins, polyamides, polyurethanes, and polyphenylenesulfides. In the invention, especially preferred are polyesters as theyhave good dimensional stability and good mechanical properties and theydo not substantially absorb visible light.

Examples of the polyesters are polyethylene-terephthalate (hereinafterreferred to as PET), polyethylene 2,6-naphthalenedicarboxylate(hereinafter referred to as PEN), polypropylene terephthalate,polybutylene terephthalate, and poly-1,4-cyclohexylenedimethyleneterephthalate. Needless-to-say, these polyesters may be homopolymers orcopolymers, but are preferably homopolymers. For the copolymers, thecomonomer includes, for example, aromatic dicarboxylic acids, aliphaticdicarboxylic acids, alicyclic dicarboxylic acids, and diols having from2 to 15 carbon atoms. Their examples are isophthalic acid, adipic acid,sebacic acid, phthalic acid, sulfonate base-containing isophthalic acid,and their ester-forming compounds, diethylene glycol, triethyleneglycol, neopentyl glycol, and polyalkylene glycols having a molecularweight of from 400 to 20,000.

These polyesters may contain various additives such as heat-resistantstabilizer, antioxidant stabilizer, organic lubricant, organic andinorganic fine particles, light-proofing agent, antistatic agent,nucleating agent and coupling agent, not interfering with the desiredeffects.

One preferred aspect is described below, in which polyester is used asthe white film base material. To whiten the polyester film for example,employable are a method of adding thereto various types of white dye orpigment; and a method of forming fine voids inside it as in the above.For attaining better results in the invention, preferred is the methodof forming fine voids inside the film. For forming such fine voidsinside it, for example, employable are (1) a method of adding a foamingagent to the resin to thereby make the resin film foamed by heat in thestep of extrusion or film processing or foamed through chemicaldecomposition; (2) a method of adding a vapor such as carbon dioxide ora vaporizable substance to the resin during or after its extrusion tothereby make the resin film foamed; (3) a method of adding athermoplastic resin not miscible with polyester to the resin,melt-extruding the resin mixture, and monoaxially or biaxiallystretching the resin film; and (4) a method of adding organic orinorganic fine particles to the resin, melt-extruding the mixture, andmonoaxially or biaxially stretching the resin film. The fine voidsformed in the film are to increase the reflective interface therein, forwhich, therefore, preferred is the method (3) or (4).

The size of the voids formed in the methods as above (this is the sizeof the cross section of the voids cut in the direction of the thicknessof the film) preferably falls between 0.5 μm² and 50 μm², morepreferably between 1 μm² and 30 μm², in view of the increased brightnessof the film. The cross-sectional profile of the voids may be circular oroval. Preferably, the film is so constituted that at least one voidexists everywhere in its vertical direction running from the top face tothe back face thereof When the film is formed into a reflecting sheet,the light from a light source enters it through the film surface, and itis the best that all the incident light having reached the reflectingsheet is entirely reflected by the voids inside the film. In fact, somelight will pass through the film, and it shall be a light loss. Tocompensate, the surface of the film opposite of the surface thereof thatreceives light (facing the light source) is preferably coated with metalsuch as aluminum or silver through vapor deposition. In addition, forreducing the light loss through the fine voids-containing polyesterfilm, it is also desirable that the surface of the film is coated with alayer that contains fine voids formed by organic or inorganic fineparticles. The surface layer may be formed by co-extruding a polyesterresin that contains organic or inorganic fine particles, along with theresin for the fine voids-containing film, followed by stretching theresulting composite film at least in one direction. Preferably, thevoids in the surface layer are smaller than those in the inner layer ofthe composite film for increasing the brightness of the film. The ratio(void size in surface layer/void size in inner layer) is notspecifically defined, but preferably falls between 0.05 and 0.8, morepreferably between 0.07 and 0.7, most preferably between 0.1 and 0.6.The void size can be controlled by controlling the size of the particlesto be added to film-forming resins.

Now described hereinunder are resins not miscible with polyester resin,and the organic or inorganic particles to be added to the surface layerand the inner layer, which are to form voids in polyester films. Theresin not miscible with polyester film (hereinafter referred to as“immiscible resin”) is a thermoplastic resin except polyester, and thiscan disperse in polyester, forming particles therein. Preferred examplesof the resin of the type are polyolefin resins such as polyethylene,polypropylene, polybutene, polymethylpentene; as well as polystyreneresins, polyacrylate resins, polycarbonate resins, polyacrylonitrileresins, polyphenylene sulfide resins, and fluororesins. These may behomopolymers or copolymers, and two or more different types of these maybe combined for use herein. Especially preferred are resins that yield agreat critical surface tension difference from polyester and hardlydeform in heat treatment after stretching. For these, preferred arepolyolefin resins, and more preferred is polymethylpentene. The contentof the immiscible resin to be in the white film is not specificallydefined, and may be suitably determined so that the film is not brokenwhile formed and the brightness of the film can be increased by thevoids formed from the nuclei of the immiscible resin in the film. Ingeneral, it falls preferably between 3 and 35% by weight, morepreferably between 4 and 30% by weight, most preferably between 5 and25% by weight. If the content is smaller than 3% by weight, thebrightness of the film could not increase so much; but if larger than35% by weight, the film may be broken while formed.

The inorganic fine particles to be added to the inner layer and/or thesurface layer are preferably those that may be nuclei by themselves toform voids in the layers. For these, for example, usable are calciumcarbonate, magnesium carbonate, zinc carbonate, titanium oxide (anatasetype, rutile type), zinc oxide, barium sulfate, zinc sulfide, basic zinccarbonate, titanium mica, antimony oxide, magnesium oxide, calciumphosphate, silica, alumina, mica, talc, kaolin. Of those, especiallypreferred are calcium carbonate and barium carbonate that absorb littlevisible light falling within a range of from 400 to 700 nm. Particlesthat absorb visible light are problematic as the brightness of the filmcontaining them is lowered. The organic fine particles are preferablythose not fusing in melt extrusion. For these, especially preferred arecrosslinked particles of crosslinked styrene or crosslinked acryl. Theorganic fine particles may be hollow. One or more different types ofthese fine particles may be used herein either singly or as combined.The particle size is not specifically defined, generally falling between0.05 and 15 μm, preferably between 0.1 and 10 μm, more preferablybetween 0.3 and 5 μm. If their size is smaller than 0.05 μm, theparticles could not yield good voids in films; but, on the contrary, iflarger than 15 μm, it is unfavorable since the film surface may be toomuch roughened. In case where fine particles are in the surface layerand immiscible resin is in the inner layer, it is desirable that thesize of the voids formed from the nuclei of the fine particles in thesurface layer is smaller than that of the size of the voids formed inthe inner layer in order that the brightness of the film is higher. Alsopreferably, the content of the fine particles in the white film fallsbetween 1 and 30% by weight, more preferably between 2 and 25% byweight, most preferably between 3 and 20% by weight. If the content issmaller than 1% by weight, the brightness of the film could not beincreased to a desired degree; but, on the contrary, if larger than 30%by weight, the film may be broken while formed.

The specific gravity of the voids-containing white film, which is acriterion of the void content of the film, is preferably not smallerthan 0.1 but smaller than 1.3. If the specific gravity of the film issmaller than 0.1, it is problematic in that the mechanical strength ofthe film is low and the film is readily folded and is difficult tohandle. On the other hand, if the specific gravity of the film is largerthan 1.3, the void content thereof is too low, and therefore thereflectivity of the film is low and the brightness thereof may beinsufficient. In case where the film-forming thermoplastic resin ispolyester, the lowermost limit of the specific gravity of the film ispreferably 0.4. If its specific gravity is smaller than 0.4, the voidcontent of the film is too high, and it is problematic in that the filmis frequently broken while formed.

The reflecting structure for surface light source is a tabular structurethat is combined with a surface light source for light reflectionthereon, as so mentioned hereinabove. Concretely, it includes reflectingsheets for edge lights-type surface light sources for liquid crystaldisplay screens, reflecting sheets for direct back light-type surfacelight sources, and reflectors around cold cathode-ray tubes. For thereflecting structure for surface light source of the type, the degree ofwhiteness of the reflecting sheet is preferably higher for bettering thecolor tone of screens, and bluish reflecting sheets are preferred toyellowing ones. Take this into consideration, it is desirable to add afluorescent brightener to the white film. The fluorescent brightener maybe any commercially-available one, including, for example, Uvitex (byCiba-Geigy), OB-1 (by Eastman), TBO (by Sumitomo Seika), Keikol (byNippon Soda), Kayalite (by Nippon Kayaku), and Leucopoor EGM (byClariant Japan). Preferably, the content of the fluorescent brightenerin the white film falls between 0.005 and 1% by weight, more preferablybetween 0.007 and 0.7% by weight, most preferably between 0.01 and 0.05%by weight. If its content is smaller than 0.005% by weight, thefluorescent brightener may be ineffective; but if larger than 1% byweight, it is unfavorable since too much fluorescent brightener ratheryellows the white film. In the case where the white film is a compositefilm, the fluorescent brightener is more preferably added to the surfacelayer of the film.

At least one surface of the white film must be coated with a lightstabilizer-containing coating layer. The light stabilizer may be any oforganic light stabilizers of, for example, hindered amines, salicylates,benzophenones, benzotriazoles, cyanoacrylates, triazines, benzoates,oxalic anilides; and inorganic light stabilizers of, for example,sol-gel compounds. Preferred examples of the light stabilizer arementioned below. Needless-to-say, these are not limitative.

Hindered amines: bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate,polycondensate of dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylp iperidine;

Salicylates: p-t-butylphenyl salicylate, p-octylphenyl salicylate;

Benzophenones: 2,4-dihydroxybenzophenone,2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxy-5-sulfobenzophenone,2,2′,4,4′-tetrahydroxybenzophenone,2,2′-dihydroxy-4-methoxybenzophenone,2,2′-dihydroxy-4,4′-dimethoxybenzophenone,bis(2-methoxy-4-hydroxy-5-benzoylphenyl)methane;

Benzotriazoles: 2-(2′-hydroxy-5′-methylphenyl)benzotriazole,2-(2′-hydroxy-5′-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotria zole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-5′-t-octylphenol)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-amylphenyl)benzotriazole,2,2′-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol],2-(2′-hydroxy-5′-methacryloxyphenyl)-2H-benzotriazole,2-[2′-hydroxy-3′-(3″,4″,5″,6″-tetrahydrophthalimidomethyl)-5′-methylphenyl]benzotriazole,2-(2′-hydroxy-5-acryloyloxyethylphenyl)-2H-benzotriazole,2-(2′-hydroxy-5′-methacryloxyethylphenyl)-2H-benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-acryloylethylphenyl)-5-chloro-2H-benzotriazole;

Cyanoacrylates: ethyl-2-cyano-3,3′-diphenylacrylate;

Others than the above: nickelbis(octylphenyl) sulfide,[2,2′-thiobis(4-t-octylphenolato)]-n-butylaminenickel, nickelcomplex-3,5-di-t-butyl-4-hydroxybenzyl phosphate monoethylate, nickeldibutyldithiocarbamate, 2,4-di-t-butylphenyl3′,5′-di-t-butyl-4′-hydroxybenzoate, 2,4-di-t-butylphenyl3′,5′-di-t-butyl-4′-hydroxybenzoate, 2-ethoxy-2′-ethyloxalic acidbisanilide, 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-[(hexyl)oxy]-phenol.

Of the examples mentioned above, at least any of hindered amines,benzophenones and benzotriazoles are preferred, more preferably, theseare combined for use herein.

The light stabilizer to be in the coating layer is preferably mixed withany other resin component for facilitating the formation of the coatinglayer. Specifically, one preferred aspect comprises dissolving ordispersing the resin component and the light stabilizer in an organicsolvent capable of dissolving the resin component and the lightstabilizer, or in water, a mixture of two or more different types oforganic solvents, or a mixture of organic solvent/water to prepare asolution or dispersion that serves as the coating liquid for the layer.Needless-to-say, the resin component and the light stabilizer may beseparately dissolved or dispersed in such an organic solvent, water, anorganic solvent mixture or a mixture of organic solvent/water, and theresulting solutions or dispersions may be mixed in any desired ratio tobe the coating liquid. Also preferred is preparing a copolymer of thelight stabilizer component and the resin component followed by directlyusing the copolymer for the coating material. Needless-to-say, thecopolymer may be dissolved in an organic solvent, water, a mixture oftwo or more different types of organic solvent, or a mixture of organicsolvent/water to prepare a solution for the coating liquid. The resincomponent to be mixed or copolymerized with the light stabilizer is notspecifically defined. Its examples are polyester resins, polyurethaneresins, acrylic resins, methacrylic resins, polyamide resins,polyethylene resins, polypropylene resins, polyvinyl chloride resins,polyvinylidene chloride resins, polystyrene resins, polyvinyl acetateresins, and fluororesins. These resins may be used either singly or ascombined to be a copolymer or mixture of two or more of them.

Of the resin components mentioned above, preferred for the coating layerare acrylic resins or methacrylic resins, and more preferred are acrylicresins or methacrylic resins copolymerized with the light stabilizercomponent. For, copolymerizing these, it is desirable that the acrylicor methacrylic monomer component is copolymerized with the lightstabilizer monomer component.

For the light stabilizer monomer component, for example, preferred arereactive benzotriazole monomers, reactive hindered amine monomers, andreactive benzophenone monomers. Not specifically defined, thebenzotriazole monomers may be any ones having a benzotriazole skeletonand having an unsaturated bond. Their examples are2-(2′-hydroxy-5-acryloyloxyethylphenyl)-2H-benzotriazole,2-(2′-hydroxy-5′-methacryloxyethylphenyl)-2H-benzotriazole, and2-(2′-hydroxy-3′-t-butyl-5′-acryloylethylphenyl)-5-chloro-2H-benzotriazole.Similarly, the reactive hindered amine monomers and the reactivebenzophenone monomers may be any ones having a hindered amine orbenzophenone skeleton and having an unsaturated bond. Examples of thereactive hindered amine monomers arebis(2,2,6,6-tetramethyl-4-piperidyl-5-acryloyloxyethylpheny 1) sebacate,polycondensate of dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-5-acryloyloxyethylphenylpiperidine,bis(2,2,6,6-tetramethyl-4-piperidyl-5-methacryloxyethylphen yl)sebacate, polycondensate of dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-5-methacryloxyethylphenylpiperidine,bis(2,2,6,6-tetramethyl-4-piperidyl-5-acryloylethylphenyl) sebacate, andpolycondensate of dimethylsuccinate/1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethyl-5-acryloylethylphenylpiperidine.Examples of the reactive benzophenone monomers are2-hydroxy-4-methoxy-5-acryloyloxyethylphenylbenzophenone,2,2′,4,4′-tetrahydroxy-5-acryloyloxyethylphenylbenzophenone,2,2′-dihydroxy-4-methoxy-5-acryloyloxyethylphenylbenzophenone,2,2′-dihydroxy-4,4′-dimethoxy-5-acryloyloxyethylphenylbenzo phenone,2-hydroxy-4-methoxy-5-methacryloxyethylphenylbenzophenone,2,2′,4,4′-tetrahydroxy-5-methacryloxyethylphenylbenzophenone,2,2′-dihydroxy-4-methoxy-5-acryloylethylphenylbenzophenone, and2,2′-dihydroxy-4,4′-dimethoxy-5-acryloylethylphenylbenzophe none.

Examples of the acrylic or methacrylic monomer component or its oligomercomponent to be copolymerized with the light stabilizer monomercomponent are alkyl acrylates, alkyl methacrylates (the alkyl groupincludes, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl,isobutyl, t-butyl, 2-ethylhexyl, lauryl, stearyl and cyclohexyl groups),and crosslinking functional group-having monomers such as those havingany of a carboxyl group, a methylol group, an acid anhydride group, asulfonic acid group, an amido group, a methylolated amido group, anamino group, an alkylolated amino group, a hydroxyl group, and an epoxygroup. Apart from these, the light stabilizer monomer component may becopolymerized into copolymers with any of acrylonitrile,methacrylonitrile, styrene, butyl vinyl ether, maleic acid, itaconicacid and dialkyl esters thereof, methyl vinyl ketone, vinyl chloride,vinylidene chloride, vinyl acetate, vinylpyridine, vinylpyrrolidone,vinyl group-having alkoxysilanes, and unsaturated polyesters.

The copolymerization ratio of the light stabilizer monomer component andthe comonomer is not specifically defined, and the light stabilizermonomer component may be copolymerized with one or more such comonomersin any desired ratio. Preferably, however, the ratio of the lightstabilizer monomer component is at least 10% by weight, more preferablyat least 20% by weight, most preferably at least 35% by weight.Needless-to-say, the light stabilizer monomer component may form ahomopolymer. The molecular weight of the polymer is not specificallydefined, but is generally at least 5,000, preferably at least 10,000,most preferably at least 20,000 in order that the coating layer formedof the polymer could be tough. The polymer is formed into the coatinglayer, after being dissolved or dispersed in an organic solvent, wateror a mixture of organic solvent/water. Apart from these, othercommercially-available hybrid-type light stabilizer polymers, forexample, U-Double (by Nippon Shokubai) may also be used herein.

The thickness of the light stabilizer-containing coating layer is notspecifically defined, preferably falling between 0.5 and 15 μm, morepreferably between 1 and 10 μm, most preferably between 2 and 7 μm. Ifthe thickness is smaller than 0.5 μm, the durability of the coatinglayer maybe low; but, on the contrary, if larger than 15 μm, thebrightness of the layer-coated white film may lower.

The light stabilizer-containing coating layer may be directly formed onthe white film. However, when the adhesiveness between the layer and thefilm is low, the film is preferably corona-discharged or undercoated.For undercoating it, employable is any of an in-line coating process inwhich the white film is undercoated in the line where it is formed, oran off-line coating process in which the substrate is after separatelyprepared in a film-forming line, undercoated in a different coatingline. The undercoating material is not specifically defined, and may beselected from any desired ones. Its preferred examples are copolyesterresins, polyurethane resins, acrylic resins, methacrylic resins, andvarious coupling agents.

The light stabilizer-containing coating layer may be formed on the whitefilm in any desired manner. For it, for example, employable is anymethod of gravure coating, roll coating, spin coating, reverse coating,bar coating, screen coating, blade coating, air knife coating ordipping. In the case where the coating layer is cured, after formation,employable is any known curing method. For example, the coating layermay be cured through exposure to heat or to active rays such as UV rays,electron rays or radiations, or exposure to any of these combinations.In curing it, a curing agent such as a crosslinking agent is preferablyused. For forming the coating layer, employable is any of an in-linecoating process in which the white film is coated with the coating layerin the line where it is formed, or an off-line coating process in whichthe substrate is, after separately prepared and processed for crystalorientation therein in a film-forming line, coated with the coatinglayer in a different coating line.

Preferably, the white film has a mean reflectance of at least 85%, morepreferably at least 87%, even more preferably at least 90%, whenmeasured on the light stabilizer-containing coating layer thereofexposed to light having a wavelength of from 400 to 700 nm. If the meanreflectance of the white film is smaller than 85%, the screen brightnessof some liquid crystal displays comprising the white film may be low,depending on the type of the displays.

Also preferably, the white film has a degree of glossiness of at most60%, more preferably at most 50%, most preferably at most 40%, whenmeasured on the light stabilizer-containing coating layer thereof. Ifthe glossiness of the white film is larger than 60%, the screenbrightness of some liquid crystal displays comprising the white film maybe low, depending on the viewing angle to the screen.

The light stabilizer-containing coating layer may contain variousadditives not interfering with the effect. The additives are, forexample, organic and/or inorganic fine particles, fluorescentbrightener, crosslinking agent, heat-resistant stabilizer, antioxidantstabilizer, organic lubricant, antistatic agent, nucleating agent, andcoupling agent.

Adding organic and/or inorganic fine particles to the coating layer isespecially preferred, as making it easy to control the glossiness of thelayer-coated surface of the white film within the defined range asabove. For the inorganic fine particles, for example, usable are silica,alumina, titanium oxide (anatase type, rutile type), zinc oxide, bariumsulfate, calcium carbonate, zeolite, kaolin, and talc. For the organicparticles, for example, usable are crosslinked styrene and crosslinkedacryl. Preferably, the particle size of the organic and/or inorganicfine particles falls between 0.05 and 15 μM, more preferably between 0.1and 10 μm. If smaller than 0.05 μm, the effect of the particles toreduce the surface glossiness may be low; but, on the contrary, iflarger than 15 μm, it is unfavorable since the surface of the white filmmay be too much roughened and the particles may readily drop off. Thecontent of the particles in the coating layer preferably falls between0.5 and 50% by weight, more preferably between 1 and 40% by weight, mostpreferably between 2 and 30% by weight. If the content is smaller than0.5% by weight, the effect of the particles to reduce the surfaceglossiness maybe low; but, on the contrary, if larger than 50% byweight, it is unfavorable since the coating layer is difficult to formand, in addition, the surface of the white film may be too muchroughened and the particles may readily drop off.

Adding a fluorescent brightener to the coating layer is more preferred,as improving the whiteness and the appearance of the layer-coated whitefilm. The fluorescent brightener may be the same as those mentionedhereinabove that can be added to the white film. The fluorescentbrightener content of the coating layer preferably falls between 0.01and 2% by weight, more preferably between 0.03 and 1.5% by weight, mostpreferably between 0.05 and 1% by weight. If its content is smaller than0.01% by weight, the fluorescent brightener may be ineffective; but iflarger than 2% by weight, it is unfavorable since the coating layer maybe rather yellowed and the durability thereof may lower.

Preferably, the thickness of the white film falls between 10 and 500 μm,more preferably between 20 and 300 μm. If the thickness thereof issmaller than 10 μm, the reflectivity, the whiteness and the appearanceof the white film may not reach the practical level, and the white filmmay be difficult to handle. On the other hand, if thicker than 500 μm,the white film may be too heavy and may be therefore unsuitable for areflecting structure for surface light sources to be in liquid crystaldisplays and, in addition, its cost may increase. In a case where thewhite film is a composite film, the ratio of surface layer/inner layerthereof preferably falls between 1/30 and 1/3, more preferably between1/20 and 1/4. In a case where the composite film has a three-layeredstructure of surface layer/inner layer/surface layer, the ratio shall berepresented by the total of the two surface layers/inner layer.

One example of producing the white film for a reflecting structure forsurface light sources is described below, to which, however, the exampleis not limiting.

A composite film-forming device equipped with an extruder A and anextruder B is prepared. To the extruder A, fed is a mixture of 85 partsby weight of dry PET chips, 15 parts by weight of polymethylpentene, and1 part by weight of polyethylene glycol having a molecular weight ofabout 4000. To the extruder B, fed is a mixture of 90 parts by weight ofPET, 10 parts by weight of calcium carbonate particles having a meanparticle size of about 1 μm, and 0.03 parts by weight of a fluorescentbrightener. Needless-to-say, the raw material components to be fed intothe extruders A and B may be previously mixed and pelletized. Theextruders A and B are heated at 280 to 300° C., and the mixtures thereinare melted and extruded out in such a manner that the inner layer of themelt from the extruder A is sandwiched between two surface layers of themelt from the extruder B to thereby constitute a composite laminatesheet. Then, the thus-extruded sheet is solidified on a chill drumhaving a surface temperature of from 10 to 60° C. In this step,preferably, the drum is electrostatically charged so that the sheet canbe airtightly held thereon and can be therefore uniformly solidifiedthereon. Then, the thus-cooled and solidified sheet is led to rollsheated at 70 to 120° C., by which the sheet is stretched about 2 to 5times in the machine direction, and thereafter it is cooled around rollskept at 20 to 40° C. In the continuous line, the film is then led into atenter while its edges are held by clips, in which it is preheated at 90to 120° C. and stretched 3 to 6 times in the lateral direction. Still inthe continuous line, the thus-stretched sheet is led into a zone heatedat 180 to 230° C., in which it is thermoset for about 3 to 20 secondsand then cooled to 40° C. or lower. In that manner, the white film ofthe invention is fabricated. To one surface of the thus-fabricated whitefilm, applied is a coating liquid prepared by mixing a UV-absorbingcompound, a light stabilizer and a resin in a predetermined ratio, andthe thus-coated film is then dried.

The white film for a reflecting structure for surface light sources ofthe invention, thus obtained in the manner as above, has good initialbrightness and is aged little even in long-term use, and it is effectivefor keeping the brightness of liquid crystal display screens.

[Methods of Measuring and Evaluating Characteristics]

-   (1) Mean Void Diameter:

The cross section of the film is observed at 3,000 to 200,000magnifications, using a transmission electronic microscope, HU-12 Model(by Hitachi). With the voids seen therein being marked, the TEM pictureis analyzed by the use of a high-vision image analyzer, PIAS-IV (byPIAS). Concretely, 100 of the voids are converted into true circles, andthey are averaged to obtain the mean void diameter of the voids.

-   (2) Specific Gravity:

The film is cut into pieces of 50 mm×60 mm each. Using a high-precisionelectronic gravimeter SD-120L (by Mirage Trade), the piece sample ismeasured according to the method A (underwater displacement) of JISK-7112. In measuring it, the temperature is 23° C. and the humidity is65% RH.

-   (3) Glossiness:

Using a digital varied-angle glossmeter, UGV-5B (by Suga TestInstruments), the film is measured according to JIS Z-8741, with itswhite polyester layer (A) being exposed to light. The incident angle is60°, and the light intercepting angle is 60°.

-   (4) Mean Reflectance:

Using a spectrocolorimeter, SE-2000 Model (by Nippon Denshoku Kogyo),the spectral reflectance of the film is measured at intervals of 10 nmwithin a range of from 400 to 700 nm, according to JIS Z-8722. The dataare averaged, and it indicates the mean reflectance of the film.

-   (5) Brightness of Surface Light Source:

As in the device of FIG. 1, a dot pattern 15 is printed on a transparentacrylic light guide plate 14 of 2 mm thick. On the side of the dotpattern printed on the transparent acrylic light guide plate, set is areflecting sheet 11 made of a film sample; and on the other sidethereof, laminated is a diffuser 13 made of a semi-transparent sheet. Acold cathode-ray tube 16 of 6 W fluorescent lamp is fitted to one end ofthe transparent light guide plate 14, and it is covered with a reflector12 as in FIG. 1. The fluorescent lamp is put on, and the brightness(cd/m²)of this structure is measured on the side of the diffuser 13,using a digital photometer J16 and a brightness probe J6503 (byTectronics). The light interceptor of the brightness probe fitted to thephotometer is applied vertically to the diffuser 13. 9 points in 9uniform sections of the diffuser surface are measured three times each,and the data are averaged.

-   (6) Mean Reflectance and Brightness After Durability Test:

Using a UV aging promotion tester, I-Super UV Tester SUW-W131 (byIwasaki Electric), film samples are forcedly exposed to UV rays underthe condition mentioned below.

Condition of UV Irradiation:

-   -   Illuminance, 100 mW/cm²; temperature, 60° C.; humidity, 50% RH;        time for irradiation, 8 hours.

After thus exposed to UV rays, the sample films are measured todetermine the mean reflectance and the brightness thereof, according tothe methods (4) and (5).

EXAMPLES

The films are described with reference to the following Examples andComparative Examples, to which, however, the films are not limited.

Example 1

Raw material mixtures mentioned below were fed into a compositefilm-forming device equipped with an extruder A and an extruder B.

Extruder A:

90 parts by weight of PET chips dried in vacuum at 180° C. for 4 hours,10 parts by weight of polymethylpentene, and 1 part by weight ofpolyethylene glycol having a molecular weight of 4000.

Extruder B:

100 parts by weight of PET chips containing 15% by weight of bariumsulfate particles having a mean particle size of 1 μm, and dried invacuum at 180° C. for 4 hours, and 3 parts by weight of PET master chipscontaining 1% by weight of a fluorescent brightener, OB-1 (by Eastman),and dried in vacuum at 180° C. for 4 hours.

In the extruders A and B, the mixtures were separately melted andextruded out of them at 290° C. in such a manner that the melt from theextruder A could be an inner layer sandwiched between two outer layersof the melt from the extruder B, and the resulting laminate melt wasextruded out through a T-die into a sheet. The thickness ratio, B/A/B ofthe composite film was 5/90/5. This was cast onto a mirror-face chilldrum having a surface temperature of 20° C. to be a non-stretched sheet.The sheet was pre-heated by rolls heated at 90° C., and then, at 95° C.,it was stretched 3.5 times in the machine direction. One surface of themonoaxially-stretched sheet was corona-discharged in air, and thencoated with a polyurethane emulsion, AP-40 (by Dai-Nippon Ink) to havethereon a coating layer having a dry thickness of 0.3 μm. Next, thesheet was led into a tenter heated at 105° C., with its edges being heldby clips, in which water was removed from the coating layer. In thecontinuous line at 110° C., the sheet was then stretched 3.5 times inthe lateral direction. Still in the continuous line, the sheet wasthermoset at 215° C. for 8 seconds. The white film thus fabricated hadan overall thickness of 188 μm.

The polyurethane-coated surface of the white film was coated with acoating liquid containing a light stabilizer, U-Double UV6010 (by NipponShokubai) to form thereon a coating layer having a dry thickness of 5μm. Thus coated, this was dried in hot air at 150° C. for 2 minutes. Thethus-fabricated white film for a reflecting structure for surface lightsources was yellowed little in the durability test, and its meanreflectance reduction and brightness reduction were small, as in Table1.

Comparative Example 1

The white film prepared in Example 1 was directly tested for itsapplicability to the white film in the reflecting structure for surfacelight source illustrated. Its initial brightness was good, but itsdurability was extremely poor. Concretely, it was greatly yellowed inthe durability test, and its mean reflectance reduction and brightnessreduction were noticeable.

Examples 2 to 5

A white film was prepared in the same manner as in Example 1. Thepolyurethane-coated surface of the white film was coated with the samelight stabilizer-containing coating liquid as in Example 1 to formthereon a coating layer having a dry thickness of 1 μm (Example 2), 3 μm(Example 3), 7 μm (Example 4), or 10 μm (Example 5). These samples wereall superior to the sample of Comparative Example 1, though thedurability of the samples of which the thickness of the coating layerwas smaller than that of the sample of Example 1 (Examples 2 and 3)lowered in some degree and the initial brightness of the samples ofwhich the thickness of the coating layer was larger than that of thesample of Example 1 (Examples 4 and 5) lowered in some degree.

Example 6

A white film was prepared in the same manner as in Example 1. Thepolyurethane-coated surface of the white film was coated with a coatingliquid having the composition mentioned below to form thereon a lightstabilizer-containing coating layer, in the same manner as in Example 1.The thus-fabricated white film for a reflecting structure for surfacelight sources was yellowed little in the durability test, and its meanreflectance reduction and brightness reduction were small, as in Table1.

(Coating Liquid for Forming Coating Layer)

U-Double UV714 (40% solution by Nippon Shokubai) 10.0 g Sumidule N3200(curing agent, by Sumitomo-Bayer Urethane)  0.5 g Ethyl acetate/toluene(1/1 by weight) 12.0 g

Examples 7, 8, 9

A white film was prepared in the same manner as in Example 6. Thepolyurethane-coated surface of the white film was coated with a coatingliquid that had been prepared by mixing 20.0 g of the coating liquid inExample 1 with 0.13 g (solid content, 3% by weight), 0.21 g (solidcontent, 5% by weight) or 0.3 g (solid content, 7% by weight) ofinorganic fine particles of silica powder (Fuji Silicia's SYLOPHOBIC100) with stirring, to thereby form on that surface a lightstabilizer-containing coating layer in the same manner as in Example 6.The thus-fabricated white film for a reflecting structure for surfacelight sources had good mean reflectance and good brightness, and wasyellowed little in the durability test, and its mean reflectancereduction and brightness reduction were small, as in Table 1.

Comparative Example 2

A single-layer film-forming device equipped with an extruder A alone wasused herein. PET chips containing 10% by weight of anatase-type titaniumoxide particles having a mean particle size of 0.15 μm were fully driedin vacuum, fed into the extruder, and melt-extruded at 290° C. into asheet. This was cast onto a chill drum at 20° C. to be a non-stretchedsheet. The sheet was stretched in the machine direction and in thelateral direction, and then thermoset into a white film having athickness of 188 μm, in the same manner as in Example 1. The film wascoated with a light stabilizer-containing coating layer, in the samemanner as in Example 1.

As in Table 1, the coated film had few voids therein, and its initialreflectance and brightness were low.

TABLE 1 Thickness of Light Content of stabilizer-con- Inorganic FineInitial Characteristics After Durability Test Tested taining Particlesin Mean Void Diameter Specific Mean Mean Matters Coating Layer coatinglayer Inner Layer Surface Layer Gravity Glossiness ReflectanceBrightness Reflectance Brightness unit μm wt. % μm μm — % % cd/m² %cd/m² Example 1 5 0 28 8 0.83 93 89 561 87 545 Co. Ex. 1 — — 28 8 0.8325 91 573 64 334 Example 2 1 0 28 8 0.83 90 90 567 78 497 Example 3 3 028 8 0.83 92 90 564 83 525 Example 4 7 0 28 8 0.83 95 88 554 87 543Example 5 10  0 28 8 0.83 95 86 538 85 531 Example 6 5 0 28 8 0.83 93 86563 84 548 Example 7 5 3 28 8 0.83 57 87 566 85 550 Example 8 5 5 28 80.83 41 88 571 85 552 Example 9 5 7 28 8 0.83 30 89 574 86 554 Co. Ex 25 0 no void — 1.43 71 76 458 74 436

INDUSTRIAL APPLICABILITY

Exposed to a light source, the white film for a reflecting structure forsurface light sources is aged little with time. When built in liquidcrystal displays, it ensures good image quality and brightness of thedisplay screens for a long time. Therefore, the white film is favorableto reflecting sheets and reflectors in edge light-type and direct backlight-type surface light sources for liquid crystal display screens.

1. A white film for a reflecting structure for surface light sourcescomprising a film containing voids and a light stabilizer-containingcoating layer formed on at least one surface of the white film, whereinthe coating layer comprises mainly a copolymer of a resin component andat least 25% by weight, based on the weight of the copolymer, of a lightstabilizer component that includes at least a hindered amine.
 2. Thewhite film as claimed in claim 1, wherein the resin is an acrylic ormethacrylic resin.
 3. The white film as claimed in claims 1 or 2,wherein the light stabilizer further includes at least one ofbenzotriazoles and benzophenones.
 4. The white film as claimed in claim1, of which the mean reflectance is at least 85%, measured on the lightstabilizer-containing coating layer thereof exposed to light having awavelength of from 400 to 700 nm.
 5. The white film claimed in claim 1,of which the degree of glossiness is at most 60%, measured on the lightstabilizer-containing coating layer thereof.
 6. The white film as claimin claim 1, of which the white film is formed of a resin compositionconsisting essentially of polyester.
 7. The white film as claimed inclaim 1, in which the voids are formed through melt extrusion of amixture of a polyester resin, and a resin not miscible with thepolyester resin and/or organic or inorganic fine particles, followed bystretching the film in at least one direction.
 8. The white film asclaimed in claim 1, of which the white film is a composite film.
 9. Thewhite film as claimed in claim 8, of which composite layers of the whitefilm contain inorganic fine particles and have voids formed from nucleiof fine particles therein.
 10. The white film as claimed in claims 8 or9, of which the white film is a composite film that contains voids inboth the surface layer and an inner layer thereof, and in which the meandiameter of the cross section of the voids is smaller in the surfacelayer than in the inner layer.
 11. The white film as claimed in claim 1,in which the coating layer additionally contains organic and/orinorganic fine particles.
 12. The white film as claimed in claim 1, inwhich the coating layer and/or the white film additionally contains afluorescent brightener.